Long time ago I developed a CCS board which provides full flexibility in terms of FETs/MOSFETs used. It was a 2-terminal CCS, a very well known circuit.
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Tag: CCS
Tail CCS PCB test
A belated test of this simple, yet effective PCB. I made it as small as possible, however in order to provide flexible connections, it’s actually double the size. Still at 4 x 4 cm is small enough.
Swenson+ tests with SMPS
Here is a test you might want to take into account. I own an HGC-320-700A SMPS which only allows for maximum current trimming, but output voltage is fixed. In order to avoid unnecessary heatsink, I will suggest using an RC before the Swenson where the R is a big Alu clamped wirewound resistor bolted to the chassis. Just leave 50V for the regulator which may dissipate a lot anyway depending on the output current.
There is no caps below and was stable enough. I could have added an output cap but I didn’t on this test. You can see how all the HF noise is trimmed out! About 40 to 60dB 🙂
The 44mA and 88mA traces are the SMPS feeding the power resistor directly.
This can be improved further with a (mandatory) output cap. A nice 30uF oil cap should do well.
I have offered some pairs built and tested to builders using the prototype boards. I may do a couple more if you are interested
CCS PCB Ready
I received the first batch of PCBs after several tweaks during the prototype phase. I’m very pleased with the result:
Flexible CCS board prototype
I’ve been prototyping a flexible CCS PCB. The intent is to provide a cascoded FET pair with some interesting features:
- The lower FET can be multiple devices depending on the choice of reverse capacitance and transconductance. These include jFETs and depletion MOSFETs like the 2SK170, J310, BF862 and of course DN2540. For this purpose several pads are provided for SMD devices as well as TO-92 ones, just like the gyrator PCB. A protection Zener diode between drain and source can be soldered when using low VDSS devices.
- There is either a string of trimpot plus a resistor to set the CCS current manually during test given the variance in the FET parameters. There is also an option to put a fixed resistor.
- There is a mu-output connection provided.
The board is very flexible and can be used for multiple purposes:
- shunt regulators (including VR valves)
- Anode load for phono preamps, drivers, LTPs, etc.
- LTP tail CCSs
I’ve been running some tests with excellent results.
If there is interest, I will run a batch of PCB to offer to the DIY community.
Cheers
Ale
Pentode driver with gyrator load
If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:
The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR).
The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please.
This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.
Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.
As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.
Cheers
Ale
Slew Rate (Part IV) and the Tale of the three Source Followers
Some of you may be a bit fed up already with these slew rate posts, however I find this fascinating as is taking me through different routes of experimentation.
On my last tests, I abused the DN2540 to an extent which meant the dead of it. So I ended up adding the appropriate back to back protection zeners on the gate:
Continue reading “Slew Rate (Part IV) and the Tale of the three Source Followers”
Gyrator bias discussion
A very interesting point was raised on the 4P1L DIYAudio thread around the gyrator circuit using CCS and whether a simple resistor divider was better than the CCS due to the LND150 temperature drift.
I’ve tried both options and I’d say I prefer the CCS despite the variation with temperature for the following reasons (which people may well disagree):
- It’s true the LND150 varies a lot with temperature (see attached), however if it’s operated at low current (e.g.<500μA) the variation is small. In a cascoded pair for this circuit the drift in the output voltage is small. Simulated in Spice I get about 6.35mV/°C. The resistor divider will be better of course but you need a smaller values to reduce impact of dR/dT. This creates another problem which is the reduced PSR. With a compromise divider to balance idle current and PSR you can get 5 times less variation with temperature in the circuit under discussion – see below (e.g. 1.4mV/°C)
- For a smaller value of resistor divider the PSR is impacted and significantly lower than the CCS. If you don’t have a well filtered supply, the PSR benefits of the gyrator will be reduced due to this. For example, I did some quick comparisons by simulating my 01a preamp. I used a 235KΩ/220KΩ and a 23K5Ω/22KΩ divider options with a typical film decoupling cap of 4.7μF.
The PSR of the CCS is above 100dB whilst the PSR of the resistor divider goes from 56dB (235KΩ/220KΩ divider) down to 37dB (23K5Ω/22KΩ divider).
In practice, I implemented two different circuits as I had a shunt regulator before when I had a resistor divider and now I don’t have any shunt regulator but I use the CCS version.
Looking at the output PSR as the gyrator provides additional rejection to noise. The resistor divider PSR is about 73dB and CCS is 30dB better anyway
The voltage variance is really small with temperature and this circuit in particular isn’t affected by such small drift in my view
Testing the line stage
Introduction
I couldn’t resist the temptation to try and build quickly the SLCF design proposed here. It was question of building a simple PCB for the tail CCS and the top MOSFET follower. Wiring it then point-to-point could be done in a matter of minutes and a “rat nest” was built fast enough to enjoy this learning experience.
The usual challenges we face when breadboarding circuits
One of the challenges we face when building a cathode follower with a high-gain / transconductance valve is that it can easily oscillate widely into VHF. So we then are a bit more precocious when building the test jig and “try” to have short connections (something which I didn’t do), add some ferrite beads to anode, grid and screen. Also some grid/screen stopper resistors (e.g. 300Ω) are always very useful. If you pay attention to this and check with an oscilloscope with sufficient bandwidth (e.g. 200MHz) you can spot out any nasty oscillation from the valve. I didn’t, thanks to the ferrite beads and stoppers.
The clear challenge of the SLCF is establishing the correct bias point on the top follower due to the high value of the resistor divider and the high-variance we typically get on the VGS(th) of the MOSFETs.
High-value resistors are available on 1% but the variance on the FET defeat the purpose of accurately building the resistor divider.
More on CCS
As I continue with my design of CCS to be used on my next designs as part of the supply filtering stage, I looked at testing the performance of my latest CCS using the following circuit:
The limitations I have currently is that my waveform generator can do 7Vrms maximum and in low frequency the existing noise level will be the limitation clearly. As suggested by Gary Pimm, adding a battery operated differential preamp at the point of test will be a great way of raising the low level signal from the sound card interface noise floor. That, will be for a future day. I just want to see how well the CCS performs.
I set the CCS to 30mA and measured attenuation from 50Hz to 30kHz. The results are quite encouraging despite the lack of pre-amplification:
The real life CCS is not that great as in the simulation. There is about 10dB difference with the Spice simulation. You can see that I can measure below -130dB attenuation without a pre-amplifier. Still is quite good, more than what we need for.
The CCS is operating to the level of what I need, so test passed 🙂